The present invention relates to the manufacture of industrial yarn, for example, for reinforcement of rubber objects such as automobile tires. It is important that industrial yarns have good thermal dimensional stability, i.e., high modulus (HM) and low shrinkage (LS). For example, for use in the production of tire cords, industrial yarns must have a hot air shrinkage as low as possible, in no event as large as 3.4%. The same is true for production of conveyor belts and rubber hoses, in which similar low shrinkage is necessary to obtain satisfactory results in the modeling step. In order to improve driving comfort provided by tire cords formed of industrial fibers, the modulus should be greater than 30 cN/tex LASE (load at specified elongation) 5%. Accordingly, the industrial yarns manufactured according to the invention are referred to herein as HMLS yarns.
To enhance durability so as to resist severe application conditions encountered, e.g., by large conveyor belts and large tires, industrial yarns must have good fatigue resistance.
It is also necessary that industrial yarns for use in reinforcement (e.g., rubber reinforcement) applications have tenacity in excess of 60 cN/tex. Accordingly, industrial yarns are of linear densities of at least 1000 dtex (preferably at least 1300 dtex) so as to provide adequate breaking tenacity/yarn.
Another important property of industrial yarns is a low elongation at break in comparison to textile yarns.
The HMLS yarns manufactured according to the invention may be processed in a known way into reinforcing cord for automobile tires, and this cord may, in a way which is also known, be provided with a bonding agent, the so-called dip, so that a "dipped cord" is formed. HMLS yarns having said properties may be manufactured in a known way by spinning PETP at a speed of about 4000 m/min into an as-spun yarn of a high degree of orientation, which should thereafter be drawn at a ratio of about 2. This known method has the disadvantage that it cannot be carried out in a single step. For, in that case, the winding speed at the end of the process would have to be at least 6000 m/min. Such a winding speed of industrial yarn in large production units makes demands on the winding equipment which cannot be met, because there are no commercially available winding units capable of winding yarns of linear density greater than 1000 dtex at speeds above 6000 m/min. As a result, in actual practice, a two-step process is used, in which spinning and drawing of the filaments are conducted separately, raising the costs of production.
For a single-step process it would be desirable to provide a technique by which a lower spinning speed in the range of 1500 to 4000 m/min can be selected, whereafter the as-spun industrial yarn thus obtained can be drawn and wound in such a way as to result in a speed of not more than 6000 m/min. It appears that at the usual relative viscosity of the polymer of about 2, the as-spun yarn will display a preorientation which is too low and the desired HMLS properties are not obtained.